1. Field of the Invention
The present invention relates to a toner used for a developer for developing an electrostatic latent image in electrophotographies, electrostatic recording, electrostatic printing, etc., and more particularly to an electrophotographic toner, developer and apparatus used for a copier, laser printer and plain paper facsimile using a direct or indirect electrophotographic image forming method. In addition, the present invention relates to an electrophotographic toner, developer and apparatus used for a full color copier, full color laser printer and full color plain paper facsimile using a direct or indirect electrophotographic image forming method.
2. Discussion of the Background
Typically, a developer used in electrophotography, electrostatic recording and electrostatic printing and the like is adhered to an image bearer such as photoreceptors on which a latent image is formed in a developing process; transferred on a transfer medium such as transfer sheets in a transferring process; and fixed on the transfer sheet in a fixing process. As the developer for developing the latent image formed on the surface of the image bearer, a two-component developer including a carrier and a toner and a one-component developer (magnetic or nonmagnetic toner) which does not need a carrier are known.
In the two-component developing method, toner particles adhere to the surface of the carrier and the developer deteriorates. In addition, the concentration of the toner in the developer decreases since only the toner is consumed, and the mixing ratio of the toner and the carrier has to be maintained at a fixed ratio. Therefore, there is a disadvantage that the developing device is enlarged. On the other hand, the one-component-developing method does not have this disadvantage and the image developer can be downsized, and is now prevailing.
Recently, office automation and colorization in an office are progressing. Opportunities increase, in which not only copies of just letters but also hundreds of copies including graphs, etc. formed by personal computers are produced by printers for presentation. The produced images are mostly solid images, line images and halftone images. In accordance with this trend, market demands for image quality are changing and demands for high reliability of image are further increasing.
Conventionally, there is a magnetic one-component developing method using a magnetic toner and a non-magnetic one-component developing method using a non-magnetic toner in the electrophotographic process using the one-component developer. The magnetic one-component developing method is mostly used for compact printers recently, in which a developer bearer including a magnetic field generating means such as magnets bears a magnetic toner including a magnetic material such as a magnetite, and in which a layer-thickness regulating member forms a thin toner layer on the developer bearer for development.
On the other hand, in the non-magnetic one-component developing method, a toner supply roller is pressed against the developer bearer to supply the toner onto the developer bearer which electrostatically bears the toner and a layer-thickness regulating member forms a thin toner layer on the developer bearer for development. This method has an advantage of being usable for colorization because of not including a colored magnetic material, and is mostly used for compact full color printers recently, which are lightweight and low cost because of not using a magnet in the developer bearer.
However, the one-component developing method still has many points to be improved. The two-component developing method uses a carrier as means of charging and transporting the toner, and the toner and carrier are transported to the developer bearer after they are sufficiently agitated and mixed in the image developer. Therefore, the toner can be stably charged and transported for a long time and the two-component developing method can be easily used for a high-speed developing device.
Compared with the two-component developing method, in the one-component developing method, defective charge and transport of the toner due to a long-time use and high-speed development tend to occur since the method does not have stable charge and transport means like the carrier. Namely, in the one-component developing method, a contact and friction charge time between the toner and friction charge members such as layer-thickness regulating members is so short that the toner having low charge and reverse charge tends to increase more than the toner of the two-component developing method using the carrier.
Particularly in the non-magnetic one-component developing method, in which ordinarily at least one toner transport member transports the toner (developer) and an electrostatic latent image formed on a latent-image bearer is developed by the transported toner, the thickness of the toner layer on the surface of the toner transport member has to be as thin as possible.
This is same for the two-component developing method in which a carrier having quite a small diameter is used. In addition, particularly when a toner having high electric resistance is used as a one-component developer, the thickness of the toner layer has to be significantly thin since the toner has to be charged by the developing device. This is because when the toner layer is too thick, only the surface thereof is charged and the toner layer cannot be uniformly charged. Therefore, the toner needs to have a quicker charge speed and to keep an appropriate charge quantity.
Conventionally, a charge controlling agent is optionally included in a toner in order to stabilize the charge of the toner. The charge controlling agent controls and maintains the friction charge quantity of the toner. Specific examples of the negative charge controlling agents include monoazo dyes, salicylic acids, naphthoic acids, metallic salts and metal complex salts of dicarboxylic acids, diazo compounds, boric complex compounds, etc. Specific examples of the positive charge controlling agents include quaternary ammonium salt compounds, imidazole compounds, nigrosin, azine dyes, etc.
However, many of these charge controlling agents have colors and cannot be used for a color toner. In addition, some of these agents do not have good solubility with a binder resin and the agents on the surface of the toner, which largely affect the charge thereof, easily leave from the surface thereof. Therefore, charge irregularity of the toner, and toner filming over a developing sleeve and a photoreceptor tend to occur. Accordingly, although images having good quality can be produced at the beginning, the image quality gradually changes and background fouling and image irregularity occur. Particularly, when the charge controlling agent is used for a toner for full color copier producing continuous images while the toner is supplied to the copier, the charge quantity of the toner decreases and the color tone becomes noticeably different from that of the initial image. In addition, an image forming unit called as a process cartridge has to be changed quickly only after several thousand images are produced, which is against environment protection and gives troubles to users. Further, most of the units include heavy metals such as chrome and are becoming problems lately in view of safety.
In order to improve the above-mentioned problems, Japanese Laid-Open Patent Publications Nos. 63-88564, 63-184762, 3-56974 and 6-230609 disclose a resin charge controlling agent which improves the solubility with a binder resin, the transparency of a fixed toner image and the safety. Since these resin charge controlling agents have good solubility with the a resin, the resultant toner has good chargeability and transparency. However, these resin charge controlling agents have disadvantages that the resultant toner has less charge quantity and charge speed than the toner using monoazo dyes, salicylic acids, naphthoic acids, metallic salts and metal complex salts of dicarboxylic acids. The chargeability of the toner improves if the addition quantity of the resin charge controlling agent is increased, but the fixability of the toner (low temperature fixability and offset resistance) deteriorates. Further, a toner including the resin charge controlling agent has a charge quantity easily influenced by humidity, and therefore background fouling tends to occur.
Japanese Laid-Open Patent Publications Nos. 8-30017, 9-171271, 9-211896 and 11-218965 disclose a copolymer of a monomer including an organic acid salt such as sulfonate groups and aromatic monomers having an electron absorption group. However, although the resultant toner has sufficient charge quantity because of hygroscopicity and adherence thought to be of the monomer including an organic acid salt such as sulfonate groups, the copolymer is not sufficiently dispersed in a binder resin. Therefore, charge irregularity of the toner, and toner filming over a developing sleeve and a photoreceptor are not sufficiently prevented. In addition, in order to improve the solubility with a binder resin such as styrene resins and polyester resins, a copolymer of a styrene monomer, a monomer including an organic acid salt such as sulfonate groups and an aromatic monomer having an electron absorption group; or a copolymer of a polyester monomer, a monomer including an organic acid salt such as sulfonate groups and an aromatic monomer having an electron absorption group is disclosed. However, the charge quantity of the resultant toner cannot be sufficiently maintained for a long time, and toner filming over a developing sleeve and a photoreceptor are not sufficiently prevented. Particularly, for a polyester resin and a polyol resin preferably used as a binder resin for a full color toner, the above-mentioned copolymer does not have a sufficient effect.
Demands for printers are expanding lately, and downsizing, speeding up of printing and lowering cost of the printers are progressing. Accordingly, high reliability and long life of the printers are beginning to be needed, and a toner capable of maintaining its properties for a long time is needed as well. However, the above-mentioned resin charge controlling agents are unable to maintain the charge controllability, and contaminate a developing sleeve and a developer layer-thickness regulating members such as blades and rollers, resulting in lowering the chargeability of the toner and toner filming over a photoreceptor. In addition, since development has to be performed by a small amount of a developer for a short time due to the downsizing and speeding up of printing, the developer needs to have good chargeability.